A BS digital broadcasting adopts a hierarchical modulation system and a system called burst symbol insertion that allows reception at a low CNR. A main signal is sent by frame and through time division multiplexing by using one or more modulation systems of 8PSK modulation, QPSK modulation and BPSK modulation. A burst symbol (BS) and a TMCC signal are BPSK-modulated.
As publicly known, one frame in the hierarchical transmission system is comprised of 39,936 symbols by inserting burst symbols (BS) among the TMCC signals, 8PSK main signals, . . . , QPSK main signals and the main signals as shown in FIG. 11A.
As shown in FIG. 12, a BS digital broadcasting receiver operates to convert received signals into intermediate frequency signals, perform on a complex operation circuit 11 a complex operation on I and Q signals acquired by orthogonally detecting the intermediate frequency signals by a quasi-synchronous detection system so as to demodulate them, and pass them through a roll-off filter 12 comprised of a FIR filter to convert them into symbol streams on a thinning circuit 13.
From demodulation data DI and DQ outputted from the thinning circuit 13, frame synchronization pattern detection, carrier regeneration, absolute phasing and TMCC decoding are sequentially performed as mentioned later, and a carrier regeneration phase error table according to a modulation system based on modulation system identifying signals comprised of A0 and A1 signals for the sake of identifying sections of an 8PSK modulation wave, a QPSK modulation wave and a BPSK modulation wave respectively is selected, and the demodulation data DI and DQ are received and supplied to a phase error detection circuit 15 constituting a phase comparator for detecting a phase error, from which circuit a phase error voltage required for carrier regeneration is acquired and supplied to a loop filter 17 so as to acquire a tuning voltage.
The tuning voltage outputted from a loop filter 17 is supplied to a numerical control frequency oscillator 18 from which data of sinωt and data of cosωt based on the tuning voltage is outputted from the numerical control frequency oscillator 18 so as to perform the carrier regeneration. To be more specific, the following complex operation is performed by using sin ωt and cosωt, namely the output of the numerical control frequency oscillator 18 and orthogonal detection output I and Q so as to consequently acquire demodulation data DI and DQ.       (                            DI                                      DQ                      )    =            (                                                  cos              ⁢                                                          ⁢                                                                        sin              ⁢                                                          ⁢                                                                                                        -                sin                            ⁢                                                          ⁢                                                                        cos              ⁢                                                          ⁢                                                        )        ⁢                  ⁢          (                                    I                                                Q                              )      
The demodulation data DI and DQ are supplied to an absolute phasing circuit 14 to be absolutely phased so as to match the phases to a sending end. The absolutely phased demodulation data ADI and ADQ signals are supplied to a W1 detection circuit 2 so as to detect a frame synchronization pattern (W1) from the demodulation data ADI and ADQ. The demodulation data for which frame timing is established based on the frame synchronization pattern detected on the W1 detection circuit 2 is supplied to a decoder division 3 to be decoded.
If the frame timing is established by detecting the frame synchronization pattern, time-series positions of the frame synchronization pattern, TMCC signals, a super frame identification pattern and burst symbols are found respectively and decoded in a decoding division 3. The TMCC signals outputted from the decoding division 3 are supplied to a TMCC decoding division to be decoded. A switching instruction signal based on the frame synchronization pattern detected on the W1 detection circuit 2 and a receiving CNR (when the CNR exceeds a predetermined value, it becomes “H”) outputted from the decoding division 3 and the decoded TMCC signals are supplied to a timing generating circuit 5 from which the modulation system identifying signals comprised of the A0 and A1 signals and a burst enable signal (BRTEN) indicating that a carrier regeneration division will be a burst symbol division are sent.
The A1, A0, switching instruction and BRTEN signals are as shown in FIG. 11 B, C, D and E. Moreover, in FIG. 12, a reference number 16 indicates a data processing circuit performing an AFC action. And a phase error detection circuit 15, the loop filter 17, the numerical control frequency oscillator 18 and the data processing circuit 16 are combined to constitute a carrier regenerating circuit 19 as a whole.
In the above conventional BS digital broadcasting receiver, one demodulator circuit 1 is used so that, in receiving operation when the CNR is high, the carrier regeneration is performed by performing phase error detection based on all the modulation systems (continuous reception). In addition, in the receiving operation when the CNR is low, the carrier regeneration is performed by burst-receiving a BPSK-modulated signal (burst reception).
Burst reception can be implemented, if mentioned in detail, by performing operation such as holding the output of the loop filter 17 on the carrier regenerating circuit 19. The BPSK-modulated signal is burst-received in the receiving operation when the CNR is low, so that the carrier regeneration is performed by using the phase error of the section.
However, there has been a problem that, in the case where performance of a down converter of an outdoor unit (ODU) or the like including an antenna is insufficient, reception with little fixed deterioration is difficult when performing the burst reception compared with the continuous reception due to influence of its phase noise.
For instance, if the BS digital broadcasting receiver receives the 8PSK, QPSK and BPSK modulation waves at a high CNR, it performs the continuous reception so as to implement the carrier regeneration. Here, if receiving situation changes and the CNR lowers, it becomes difficult for the BS digital broadcasting receiver to receive the 8PSK modulation wave and implement the carrier regeneration so that it performs the burst reception to implement the carrier regeneration except the 8PSK modulation section based on the switching instruction signal (see FIG. 11D). A determination of the switching is made by monitoring an error rate after trellis decoding and so on, and the switching is set on the BS digital broadcasting receiver so that it is switched at an arbitrary value.
In the case of considering the carrier regeneration around the switching between a high CNR and an intermediate CNR, the marginal CNR when performing the continuous reception is different from the marginal CNR when performing the burst reception (see FIG. 3). This point will be described based on FIG. 13 hereafter.
FIG. 13 is a diagram of which horizontal axis is phase noise and vertical axis is the marginal CNR, wherein (a) is the marginal CNR acquired from a bit error rate at 8PSK modulation reception when the carrier regeneration is performed by the continuous reception, (b) is the marginal CNR acquired from a bit error rate at 8PSK modulation reception when the carrier regeneration is performed by the burst reception performed by the demodulation data of the BPSK modulation section, (d) is the marginal CNR acquired from a bit error rate at QPSK modulation reception when the carrier regeneration is performed by the continuous reception (except the 8PSK modulation section), (e) is the marginal CNR acquired from a bit error rate at QPSK modulation reception when the carrier regeneration is performed by the burst reception performed by the demodulation data of the BPSK modulation section. Here, the marginal CNR is a limit value wherein any CNR lower than it cannot be corrected in connected sign error correction.
For instance, when the phase noise θ rms is 10 (deg), a switching point from the continuous (BPSK, QPSK and 8PSK modulation sections) reception to the burst reception is calculated at 9.5 dB (see FIG. 13 (a)) on a shift from high CNR reception to intermediate high CNR reception, and a switching point from the burst reception to the continuous reception is calculated at 13 dB (see FIG. 13 (b)) on a shift from the intermediate high CNR reception to the high CNR reception. Thus, there is hysteresis between mutual switching points of the continuous reception and the burst reception (shown by an arrow (c) in FIG. 13 for instance), and there further arises a problem that a factor of this hysteresis occurrence depends on the phase noise of the ODU. If phase noise characteristics of the ODU are known in advance, technical countermeasures can be taken, whereas it is actually impossible to predict the characteristics since the ODUs purchased by users are various. In order to solve this, it is also thinkable to detect a degree of the ODU phase noise, which is by no means easy.
In addition, when the phase noise θ rms is 10 (deg) likewise for instance, a switching point from the continuous (QPSK and 8PSK modulation sections) reception to the burst reception is calculated at 3.5 dB (see FIG. 13 (d)) on a shift from low CNR reception to intermediate low CNR reception, and a switching point from the burst reception to the continuous reception is calculated at 4 dB (see FIG. 13 (e)) on a shift from the intermediate low CNR reception to the low CNR reception. Thus, there is hysteresis between mutual switching points of the continuous reception and the burst reception (shown by an arrow (f) in FIG. 13 for instance), and there further arises the same problem that the factor of this hysteresis occurrence depends on the phase noise of the ODU.
An object of the present invention is to resolve the problems in the system of switching receiving methods and avoid the problem of uncertainty of the switching point depending on the ODU phase noise so as to provide the BS digital broadcasting receiver capable of optimum reception.